The present invention relates to a method for determination of the dew-point of a humid gas by means of a light beam reflected by a mirror. It also relates to a device for carrying out the method.
A dew-point measuring instrument of the known type is described, e.g., in the Directions for Use K-1806 (December 1986 Edition) of MBW ELEKTRONIK AG, CH-5430 Wettingen.
Dew-point measurement is typically carried out by use of a cooled mirror, which is cooled by means of a Peltier element until a dew or hoar precipitate forms on the mirror. The cooling is controlled via photo-optical means until that temperature is reached at which a steady dew or hoar layer is achieved.
The problem of time consuming dew-point determinations arises with gases of low water content. Up to ten hours or more are required for the condensed water vapor on the mirror to become detectable.
Although this unsatisfactory situation has long been known, no practical solutions are known so far for accelerating the condensation of water vapor to dew and thus shortening the measurement time connected with dew-point determinations of gases of low water content.
It is, therefore, an object of the invention to improve the known method and the associated device for dew-point determination in such a way that a substantial acceleration would result in dew-point measurement of humid gases of low water content. Another object of the invention is to ensure the fault- and interference-free operation of dew-point measurement instruments in process engineering.
According to the method of the present invention, this improvement is achieved in that, during measurement of low temperature dew-points, preferably in the range between -50.degree. C. to -95.degree. C., moisture is added for short periods of time to the gas whose dew-point is to be determined (hereinafter the "humid gas sample") streaming in the manifold. The dew point mirror is thus in contact for short periods of time with a gas containing added moisture, so that an initial dew layer is rapidly produced on the dew-point mirror.
The associated device in a dew-point measuring instrument with a cooled dew-point mirror according to the invention comprises a gas valve which is arranged in the manifold supply line for the humid gas sample, upstream of which valve there are arranged an injector pump and a humidification unit, whereby added moisture may be added to a portion of the humid gas sample.
With the method according to the invention, extra moisture is inserted during the measurement process into the stream of the humid gas sample whose dew-point is to be determined. This is done automatically and only for a very short period of time, at a preselected mirror temperature. This temperature is selected to be in the vicinity of the expected temperature of the dew-point which is to be determined accurately. When this preselected temperature is below the temperature of the dew-point to be determined, a particularly fast measurement of dew-point is assured.
The humid gas sample, whose dew-point is to be determined, as such or with added excess moisture according to the method of the present invention, (with and without the addition of excess humidity) is advantageously led to and removed from the dew-point mirror via pressure lines.
The device according to the invention for carrying out the method is designed in such a way that the humidification unit, which temporarily adds excess moisture to the humid gas sample, is directly connected to and is driven by the gas removal line via a pressure line. This has the advantage of adding the excess humidity to the streaming humid gas sample without causing an interruption in, or interference to the gas flow.
The injector pump is preferably a positive-displacement pump, particularly a piston pump arranged in a pressure-proof housing. This arrangement has the advantage that it can be used at atmospheric as well as at higher pressures. An arrangement comprising a needle valve in conjunction with a non-return valve has proved satisfactory.
A particularly simple embodiment of a humidification unit consists of a hygroscopic tube, preferably a plastic tube. Water molecules from the atmosphere diffuse into the interior of the tube where they admix with the gas streaming through. Practical experience has indicated the usefulness, for this purpose, of a commercially available plastic tube which is absolutely maintenance-free.
Other advantages and features of the present invention will become more apparent hereinafter.